Anaerobic digestion is a common method for stabilization of sludges produced during wastewater treatment. The anaerobic digestion or stabilization of sludges can be conducted under thermophilic or mesophilic conditions in a batch or a flow-through vessel, where anaerobic conditions are fostered to encourage fermentation and methanogenesis in the sludge biomass. The result is a decrease and removal of volatile solids that can cause putrefaction.
One example of a sludge digestion process is disclosed in U.S. Pat. No. 6,660,164 to Stover. In this patent, a one-step aerobic thermophilic process is described for treatment of waste streams. Other known treatment processes, such as that disclosed in U.S. Pat. No. 4,246,099 to Gould et al. The sludge treatment techniques discussed in that patent is a two-step process. In a first step, sludge is aerobically digested at a temperature from 35° C. to 75° C. Next, the sludge is anaerobically digested, preferably at a lower temperature.
Sludge treatments, like that described above, may consist of either a one-step treatment or a process that includes digestion of sludge under single-stage aerobic, single or multi-stage anaerobic or two-stage aerobic, followed by anaerobic conditions. However, these processes have several disadvantages:
1. Need for low nitrogen recycles. For a facility practicing nitrogen removal, the concentrated ammonia-nitrogen fraction in the reject liquor from anaerobic digestion needs to be treated either in a separate sidestream process or within the mainstream plant configuration, thus consuming significant capacity and operating expense (chemicals and aeration).
2. Need for improved volatile solids destruction. A decrease in volatile solids can reduce putrefaction and overall solids inventory (sludge minimization). Although anaerobic digestion can destroy volatile solids by as much as 50%, subsequent to anaerobic digestion, a significant fraction (a similar 50%) of volatile solids remain undigested. It is believed by the inventors that this undegraded volatile fraction is either inert or not accessible under anaerobic conditions. A further reduction in volatile solids can reduce putrefaction and reduce biosolids inventories (sludge minimization).
3. Need for improved dewatering. The biosolids from anaerobic digestion can consume considerable polymer demand during dewatering and produce variable cakes solids. A reduction in polymer demand and improvement to cake solids (sludge minimization) will benefit a facility by reducing operating costs.
4. Need for biosolids product quality improvements. Land application is an environmentally friendly, viable means of biosolids management and results in the recycling of nutrients and organics and thereby benefiting the soil for agriculture or reclamation. For many wastewater treatment plants in the United States, this viability is at a risk because of environmental pressures that result in a need to improve product quality-specifically reducing biosolids odors and treatment of endocrine disrupting and other trace chemicals. Reduction in odors and treatment of endocrine disrupting and other trace chemicals will improve product quality, marketability, reduce overall biosolids program risk, and increase the viability of the land application program.
Accordingly, there is a need and desire for a sludge processing and stabilization process and system that overcomes these disadvantages associated with the conventional wastewater treatment processing techniques.